The present invention relates to a method of continuously manufacturing a writing pencil or a coloring crayon by simultaneously extruding thermoplastic materials constituting successively the xe2x80x9cleadxe2x80x9d, and the material surrounding the lead, referred to below as xe2x80x9cwoodxe2x80x9d material by analogy with the composition of traditional pencils, where this material is indeed wood. More particularly, the invention relates to a triple extrusion method in which thermoplastic materials are extruded including an intermediate protective layer which is coextruded between the lead and the wood material. The invention also provides a pencil or a crayon suitable for being obtained by the method of the invention.
Continuous manufacture of a pencil or a crayon by coextruding thermoplastic materials constituting the lead material, an intermediate protective layer, and the wood material is already known from document EP 0 505 262. In that document, the intermediate protective layer is of non-expanded thermoplastic material compatible with the lead material and with the wood material and having a melting temperature which is equal to or greater than both the melting temperature of the lead material and that of the wood material. That particular choice of thermoplastic material for constituting the intermediate protective layer is for the purpose of avoiding defects due to the presence, on the outer periphery of the lead material, of bubbles of the kind observed when manufacturing pencils by dual extrusion while using a foaming or expanding agent in the wood material. Because of its melting temperature, the thermoplastic material constituting the protective layer is less fluid than the lead and wood materials. As a result it does not deform under the effect of bubbles given off at the surface of the wood material and thus constitutes a protective barrier for the lead material.
The problem which the present invention seeks to solve is of a different kind. It likewise relates to continuously manufacturing a pencil or a crayon by coextrusion of thermoplastic materials. However the presence of another type of defect has been observed, namely cracking of the lead, giving rise to frequent breaks, in particular when sharpening the pencil.
The object of the Applicant is to propose a method of manufacture which mitigates the above-mentioned drawbacks, by avoiding the appearance of such cracking.
This object is achieved by the method of the invention. It is a method of continuously manufacturing pencils or crayons which consists in coextruding a filled first thermoplastic material referred to as xe2x80x9cleadxe2x80x9d material, and around this both an intermediate xe2x80x9cprotectivexe2x80x9d layer in a thermoplastic material that is free from any expanding agent, and a second thermoplastic material containing an expanding agent and referred to as xe2x80x9cwoodxe2x80x9d material, and in cooling said coextruded thermoplastic materials.
In a manner characteristic of the invention, the thermoplastic material constituting the protective layer is determined in such a manner as to remain deformable while the lead material is shrinking, during cooling.
From the observations that the Applicant has made, it appears that cracking occurs mainly when the thermoplastic materials used for the lead material have a high coefficient of expansion giving rise, during cooling, to significant amount of longitudinal and transverse shrinkage. This applies in particular to the lead materials used in manufacturing soft lead pencils.
The thermoplastic material constituting the protective layer is preferably selected so that its solidification point is lower than the solidification point of the thermoplastic material constituting the lead material.
Thus, while the extruded materials are cooling, possibly after they have been cut transversely so as to give rise to individual pencils, the lead material solidifies earlier than the protective layer since its solidification point (i.e. the temperature at which the previously plastic or molten thermoplastic material begins to become solid) is higher than that of the thermoplastic material constituting the protective layer. The shrinkage of the lead material which occurs during solidification therefore takes place while the lead material is surrounded by the protective layer which has not yet solidified but which is still in a deformable state. By means of its deformability and its flexibility, this protective layer allows the lead material to release its internal stresses and thus to shrink without cracks appearing. Solidification proper of the protective layer occurs subsequently.
The difference between the solidification points of the lead material and of the protective layer is preferably about 10xc2x0 C. to 20xc2x0 C.
In order to lower the solidification point of the protective layer, it is preferable to use an additive constituted either by a thermoplastic elastomer or else by an ethyl-vinyl-acetate copolymer (EVA).
The invention also claims a pencil or crayon suitable for being obtained by the above-specified manufacturing method.
In known manner, such a pencil is constituted by superposing a core made of a filled first thermoplastic material referred to as xe2x80x9clead materialxe2x80x9d, a protective layer made of a non-expanded thermoplastic material, and a layer constituting an expanded second thermoplastic material referred to as xe2x80x9cwoodxe2x80x9d material. In characteristic manner, the thermoplastic material constituting the protective layer has a solidification point which is lower than that of the thermoplastic material constituting the lead material.
Advantageously, the protective layer contains a thermoplastic elastomer or an ethyl-vinyl-acetate copolymer (EVA).
As its thermoplastic material, the protective layer preferably comprises a styrene polymer. For example, the protective layer can comprise 70% to 95% of a styrene compound and 5% to 30% of a thermoplastic elastomer or an EVA copolymer.
It is also possible for the thermoplastic material constituting the protective layer to be constituted by a material which does not solidify after cooling but which is deformable when hot and which remains flexible. This can be a thermoplastic elastomer material, in particular.